Study On Agglomeration Characteristics And Counteraction Technology Of Biomass Combustion In Fluidized Bed

High content of potassium in biomass ash leads to agglomeration in the fluidized bed during combustion, which eventually propagates to partial or total defluidization of the reactor, and then subsequently unscheduled shut down of the boiler. In this thesis, intensive research on the agglomeration phenomenon is conducted with the expectation to find out the scientific basis to solve such problem. The reaction mechanism of agglomeration caused by biomass ash and typical bed materials is analyzed. A mathematical model is established to describe the agglomeration process in bio-fuel fired fluidized bed combustor. A method forecasting the agglomeration is proposed to explore new technology of counteracting agglomeration in bio-fuel fluidized bed.Melting behavior of wheat stalk ash and its mixture with quartz at high temperature was investigated with thermogravimetric analysis (TG) and differential scanning calorimetry (DSC). Model compounds were introduced for analysis, finding that there is eutectic reaction between KC1and CaCl2in wheat stalk ash and its mixture with quartz at the temperature from620℃to700℃. The molten KCl and CaCl2transfer to gas phase when the temperature ranges from800℃to900℃. At the air atmosphere, K2CO3in wheat stalk ash can react with SiO2, forming potassium silicate, the part of which in the wheat stalk melts at the temperature from901℃to1039℃. Compared with the wheat stalk ash, there is no obvious difference for the range of melting temperatures of CaCl2and KCl in the mixture of wheat stalk ash and quartz, but the initial melt temperature of silicate decreases markedly. The Melting of potassium silicate is the main reason for the agglomeration of the mixture of wheat straw ash and quartz.Melting behavior of wheat stalk ash and its mixture with quartz at high temperature was investigated with TG/DSC, polarized light microscopy, scanning electron microscopy and energy dispersive X-ray (SEM-EDX)and Multi-phase equilibrium calculation. No agglomeration is detected below850℃. At the temperature ranging from900℃tolOOO℃, however, obvious agglomeration is observed and the agglomerates solidify further as the temperature increases. The presence of potassium and calcium causes a sticky sand surface that induces agglomeration. The main components of the agglomerate surface are K2O-SiO2-Na2O-Al2O3-CaO. The agglomeration is not caused by the melting behavior of wheat stalk ash itself but due to the comprehensive results of chemical reaction and the melting behavior at high temperatures. The formation of molten silicates will directly lead to the agglomeration of the mixture of wheat straw ash and river sands.Based on the balance mechanism of the adhesive force caused by liquid bonding between two particles and the breaking force induced by bubbles in the fluidized bed, a mathematical model is established to describe the agglomeration process in bio-fuel fired fluidized bed combustor, which considers the modified Urbain model and chemical equilibrium calculation using FactSage modeling. This model accounts for the evolvement of the adhesive and breaking forces, and clearly demonstrates that the different compositions of ash, the increasing matter of liquid phase and the fluidization velocity cause defluidization in fluidized bed. In this model, it is hypothesized that the bonding stress between two particles is proportional to the mass fraction of liquid phase and inversely proportional to the diameter of particles. Viscosity of liquid phase is put forward for the first time. The defluidization time calculated by this model shows good agreement with that from the experimental data.Multiphase equilibrium calculation is performed by the FactSage to identify the melting behavior of biomass ash blended with different bed materials. The proportion and the composition of liquid phase and solid phase at different temperatures were predicted by the Equilib module of the FactSage. The calculation results are verified by polarized light microscopyand energy dispersive X-ray. A prediction method based on multiphase equilibrium calculation is set up and tested by polarized light microscopyand energy dispersive X-ray experiments. The calculation results of multiphase equilibrium show that compared with river sands, the bed materials, the clay, kaolin and stone coal ash can counteract agglomeration, which is consistent with the experimental results.The characteristics of bed agglomeration during combustion of wheat stalk pellet in a bench scale bubbling fluidized bed with four kinds of bed materials are investigated with the data from scanning electron microscopy(SEM) and energy dispersive X-ray(EDX) and multiphase equilibrium calculation. The bed materials include kaolin, coal gangue ash, fluidized bed coal slag and blast furnace slag. The results indicate that compared with the commonly used silica sands, all of the four kinds of bed materials can reduce the agglomeration of biomass ash. Especially, coal gangue ash and fluidized bed coal slag can effectively counteract the agglomeration. The high content of aluminum, sulfur and alkaline, and low content of alkali are conducive to improve the melting point of coating layer covered by the surface of the bed material pellets to prevent them from growing. Therefore, further bed agglomeration and defluisization can be prevented.